A method will be developed for the direct selection of plasmid-containing exconjugants in mating experiments between virulent and avirulent strains of Agrobacterium tumefaciens. This technique will be based upon the ability of arginine auxotrophs of A. tumefaciens to utilize octopine or nopaline as arginine sources. The genes for nopaline metabolism are known to reside on plasmid DNA. Once established, the mating technique will be used to identify and map genes involved in tumor induction as to their linkage states and presence on plasmid or chromosomal DNA. In addition plasmids of several isolates of strain B6 showing altered host specificity will be analyzed by genetics, nucleic acid hybridization techniques and by restriction endonuclease cleavage patterns in an effort to determine whether or not such altered host specificity results from alteration or deletion of plasmid sequences.